Photodetectors convert optical signals to electrical signals. More specifically, photodetectors absorb incident photons which produce electrical charge carriers. The flow of these charge carriers produces current.
Photodetectors are widely used in fiber optic communications. Photodetectors which have very low dark currents, i.e., low current flow in the absence of light, generally exhibit high signal-to-noise (s/n) ratios and, accordingly, are very desirable for use in fiber optic systems. Generally, such photodetectors are diodes having a P-I-N structure (PIN's), i.e., diodes in which the p and n regions are separated by a high-resistivity intrinsic layer, or more elaborate avalanche photodiodes (APD's).
The wavelength range of the photons used in fiber optic communications is on the order of 0.8 .mu.m to 1.6 .mu.m, although longer or shorter wavelengths may be used. Wafers comprising epitaxial structures grown on InP substrates are typically used in constructing photodetectors in the 1.3 .mu.m to 1.6 .mu.m wavelength range. These photodetectors include one or more layers of InP, InGaAs and InGaAsP compositions which are lattice matched to the underlying InP substrate, i.e., the structural arrangement of the atoms of these layers is the same as the structural arrangement of the atoms of the InP substrate. Generally, the wafers are processed into finished photodetectors using elaborate steps consisting of full-surface layering, i.e., full-surface diffusion, full-surface metallization, full-surface anti-reflection coating, and several steps of photolithography to pattern appropriate features into these full-surface metallic or dielectric layers in order to form the device. The patterns are formed by a photoresist lift-off technique or etched into the previously deposited full-surface layers by using toxic and corrosive acid mixtures or by organic solvents.
Photodetectors typically include two ohmic contacts such as a metallic n-contact on one side of the device and a metallic p-contact on the other side of the device. Accordingly, full-surface metallization followed by photolithographic patterning is conventionally used on both sides of the photodetector to form these contacts.
Furthermore, at least one side of the device must permit entry of light or radiation into the photodetector so as to enable detection thereof. Thus, an appropriate anti-reflection coating is deposited in the light or radiation entry area of the photodetector to improve the photoresponse of the photodetector to the incoming light. A full-surface anti-reflection coating is conventionally used and, again, photolithographic patterning must follow.
Unfortunately, full-surface layering is extremely inefficient and time-consuming. Separate steps of photolithography are employed to pattern appropriate features into each deposited full layer. These steps of photolithography generally follow each step of full-surface layering. Accordingly, each step of full-surface diffusion, full-surface metallization and full-surface anti-reflection coating requires a subsequent step of photolithographic patterning of each full-surface layer; the pattern being appropriate for a given photodetector design. A significant amount of time is required to perform the semiconductor processing steps of deposition, sputtering, etching, etc. required to form the photodetector.
The above-described process is conventionally utilized in the manufacture of semiconductor devices such as photodetectors and generally results in a low yield (10%-20%) of low dark current, low noise and high breakdown voltage photodetectors. Additionally, this process involves undesirable requirements such as the need for full-surface layering throughout the manufacturing steps. This results in excessive processing durations, for example, seven to ten days. In sum, the conventional process is inefficient and results in high cost semiconductor photodetectors.